What Is the Efficient Market Hypothesis?
Formalized by Eugene Fama in 1970, EMH argues that competition among rational investors drives prices to incorporate new information almost instantly. If a stock were predictably cheap, traders would buy it until the discount disappeared, so any remaining price changes should be driven by genuinely new, unpredictable information.
The hypothesis comes in graded forms. The weak form says prices already reflect all past trading data, so technical analysis cannot produce excess returns. The semi-strong form adds all public information, implying fundamental analysis of filings and news is also futile. The strong form claims even private information is priced in, a version contradicted by the profitability of illegal insider trading.
Evidence For and Against
The strongest evidence in EMH's favor is the persistent failure of professional managers to beat their benchmarks. S&P's SPIVA scorecards regularly show that well over 80 percent of active US equity funds underperform the S&P 500 over 15-year periods after fees. Fama shared the 2013 Nobel Prize for this body of work, an award he split with Robert Shiller, one of the theory's leading critics.
Critics point to documented anomalies such as momentum, the value premium, and post-earnings-announcement drift, along with episodes like the 1987 crash and the dot-com bubble that are hard to square with rational pricing. Behavioral finance attributes these patterns to investor psychology, while defenders reply that most anomalies shrink after trading costs or reflect compensation for bearing risk.
Why It Matters in Practice
EMH reshaped the asset management industry. If markets are efficient, the rational strategy is to buy low-cost index funds, and trillions of dollars have migrated to passive vehicles on exactly that logic. Every active manager's pitch is implicitly a claim about where and why the hypothesis fails, whether in small caps, distressed debt, or less-followed markets.
The concept is also a reliable interview topic. Candidates for equity research, hedge funds, and asset management should be able to define the weak, semi-strong, and strong forms, cite evidence on both sides, and articulate a coherent view on why their target firm's strategy can earn excess returns despite broadly efficient markets.
